As others have said, tube amps provide a more usable power band than most solid state amps. I've been in audio for over 40 years, and have listened to countless amps, and of this I have no doubt. Another point on most tube amps is that the output transformers allow these amps to deal with complex loads quite well. Sonically, I have always found tube amps to be less fatiguing in the mids and highs.
That said, the tradeoff is the low end. AR9s, even with a massive tube amp, will be soft in the bottom end. This is the biggest tradeoff in my view. That, and the dynamic compression that occurs at higher volume on tube amps, will lead to a choice. If recreating the energy of a concert, with gut punching bass is what you're looking for, best in class solid state is hard to beat.
That said, the tradeoff is the low end. AR9s, even with a massive tube amp, will be soft in the bottom end. This is the biggest tradeoff in my view. That, and the dynamic compression that occurs at higher volume on tube amps, will lead to a choice. If recreating the energy of a concert, with gut punching bass is what you're looking for, best in class solid state is hard to beat.